Magnetic field sensors that detect the motion of a moveable target are known. The target can be a hard ferromagnetic (permanent magnet) or a soft ferromagnetic target. Magnetic field sensors that detect the features of a ferromagnetic gear target belong to a class of sensors sometimes referred to as “gear tooth sensors,” Gear tooth sensors are key elements in engine management applications, anti-lock braking systems, transmission systems, and other industrial or automotive systems.
In this type of magnetic field sensor, the magnetic field associated with the target's mechanical profile is sensed by a magnetic field sensing element, such as a Hall element or magnetoresistive (MR) element. When the target is a soft ferromagnetic material, a direct current (DC) bias magnetic field is provided to the sensing element, typically by positioning a permanent magnet near the sensor. The DC bias magnetic field is a static, or zero-frequency, magnetic field. As the target's features pass the sensing element, the magnetic field experienced by the sensing element varies in relation to the target profile. The sensing element senses the magnetic field as it changes and produces a signal proportional to the sensed magnetic field. Typically, when the target has a repeating pattern of features (such as teeth and valleys of a toothed gear), the magnetic field signal produced by the sensor will take the form of a sinusoidal signal.
Some sensors process the magnetic field signal to generate an output signal that changes state each time the magnetic field signal crosses a threshold. Such an output signal may be used to determine information such as rotational speed for a rotating target.